The present invention concerns a process for increasing the resistance to cracking corrosion of elongate elements, particularly elongate metal elements formed by cold drawing, as well as the products obtained by this process.
It is known that certain metals are sensitive to cracking corrosion when exposed to certain aqueous corrosive media, and particularly when exposed to aqueous media containing hydrogen sulfide, so that when they are simultaneously subjected to tensile stresses their sensitivity to cracking corrosion becomes greater as these stresses become higher.
In particular, this type of corrosion occurs with cold-drawn carbon steels whose hardness after drawing exceeds the value of 22 Rockwell C, which is the case of the steels of high carbon content.
On the contrary, the steels whose hardness is lower than the above-mentioned limit value are not sensitive to cracking corrosion in aqueous medium containing hydrogen sulfide.
A typical composition of steels likely to undergo this type of corrosion is, for example,: C=0.84%; Mn=0.575%; Si=0.174%; S=0.008%; P=0.017%.
Such steels undergo a hardening treatment called "patenting" and are then drawn and cold-shaped.
An example of the mechanical characteristics thus obtained is as follows: resistance to traction: 1360 MPa, elastic limit at 0.2%; 1.280 MPa.
An important but non exclusive application of this invention consists in the protection against cracking corrosion of elongate metal elements destined to be helically wound to form armourings of flexible pipes or cables.
By experiment it has been ascertained, as a matter of fact, that in certain conditions of use, particularly in contact with agressive media such as sea water, the armourings may be damaged by the cracking corrosion, leading to the breaking of the flexible pipe or cable.
It is already known, particularly from the French Pat. No. 1,426,113, the British Pat. No. 1,054,979 and the German Pat. No. 1,227,491, to use processes for improving the resistance of the metals to corrosion, wherein said metals are subjected to mechanical coldworking surface treatments, such as sand-blasting, shot-blasting, rolling or running between roller, embossing, hammering and polishing. The so-obtained improvement results from the compression of the metal (strain hardening) in the vicinity of its surface. However, the depth of the metal affected by these treatments remains small and is generally limited to 0.1 mm. (with an intense shot-blasting, a thickness of 0.2 mm can be reached).
This depth of treatment obtained by prior processes is insufficient, since the resistance to cracking corrosion thus obtained over a small thickness does not prevent the generalized corrosion of the metal. The latter progressively destroys the surface layer subjected to compression by the previous treatment and the sensitivity of the metal to cracking corrosion then reappears.